David Starkey: This miniature is Lady Jane Grey

For decades, the female sitter in this 16th-century miniature painting remained unidentified. But after investigating the woman's jewellery, the historian David Starkey is now convinced this is the only known portrait of Lady Jane Grey painted during her lifetime to have survived.

It may have been produced when Lady Jane, the queen who ruled for nine days, was living in royal lodgings at the Tower of London where, instead of being crowned, she was executed in February 1554.

The tiny picture will go on display tomorrow as part of an exhibition of newly discovered royal Tudor portraits organised by the London art dealer Philip Mould.

The remarkable discovery is likely to fuel speculation about a painting believed to be Lady Jane Grey, but painted after her death, which was acquired by the National Portrait Gallery for £100,000 last year.